30 June 2017

An application has been made for Planning Consent for a new footbridge at Tintagel Castle in Cornwall.

The last time I featured this project was to discuss the six shortlisted competition entries back in December 2015. In March 2016, the winner was announced as Ney and Partners with William Matthews Associates. The scheme is for a new bridge to take visitors onto the Tintagel Castle promontory, a beautiful and deeply historic site. The bridge will provide access for the mobility-impaired for the first time.

You can find the full planning application online, but I've extracted some of the pertinent material to share here.

The bridge gives the appearance of being a very slender arch structure, but in fact it is formed of two giant steel cantilevers, each shaped with a parabolic curve in elevation. In theory, this means that the lower rib carries a constant force when the bridge is subject to a uniform load, allowing for an efficient use of structural steel. In practice, things are never so simple.

The lower and upper ribs each comprise two weathering steel fabricated box girders. These span 66.7m in total. The cantilevers are not quite symmetrical.

The upper ribs are parallel, with the 3.0m wide structure supporting a 2.5m wide walkway. At midspan, these ribs are a mere 175mm deep, impressively slender by any standard. The lower ribs converge towards their foundations, and are also exceptionally small, being only 140mm deep over most of their length.

The structure's strength comes from the depth of the twin cantilevers, which reaches 4.4m near the supports. The upper and lower ribs are connected by what the designers refer to as a "Thomas Telford" detail, for reasons which should be obvious. These spandrel lattices are formed from solid stainless steel bars varying in cross-section from 30mm square to 65mm square.

The structural dimensions illustrate a peculiar talent that Ney and Partners seem to have for exploiting design standards to their absolute limit, and creating structures of astonishing slenderness. The total weight of the steel structure is stated as 66 tonnes, which is quite amazing for this span. It's no surprise given the slenderness to read that the bridge's first natural frequency is 1.6 Hz (well into the vulnerable area for pedestrian excitation).

The bridge deck consists of slates placed on edge in a sand bedding layer, carried on stainless steel pans supported between the primary structural ribs. The pans have drainage scuppers in the soffit, and there is an air gap between the deck pans and the main ribs to ensure the weathering steel can weather properly.

The bridge foundations are proposed as rock anchors for both the compression (lower rib) and tension (upper rib) elements, with additional rock anchors used to stabilise the exposed cliff faces.

The gap in the middle of the bridge is nominally 42 mm, reducing to 5 mm under maximum temperatures, and increasing to 85 mm under minimum temperatures. There's clearly a degree of controversy to this particular detail, given the risk of a trip hazard or simply the discomfort caused to visitors already made anxious by height and exposure.

At competition stage I observed that significant differential deflections could also be expected when one cantilever was loaded more than the other (by pedestrians or by wind), but the planning submission makes clear that the two cantilevers are in fact connected by shear pins, in a similar manner to a twin-bascule bridge.

I think the poetic idea behind the gap justifies the problems that it creates: the intention is to make intensely apparent the sensation of stepping from the present into the past, of the division between the Tintagel Castle and the mundane world.

The designers have also sought to address the other major objection raised at competition stage, which was to the adoption of weathering steel at an exposed coastal site, where the wind will blow salt spray high above the sea. They have instituted a series of corrosion tests on steel plates exposed at the project site, and the planning submission documents make clear that if these are unsuccessful, the weathering steel will simply be substituted with conventional painted structural steel.

However, there seems to be little acknowledgement of the bimetallic corrosion issue created by the use of so much stainless steel and weathering steel connected together. This combination will tend to lead to accelerated corrosion of the weathering steel at connection points, especially if moisture and salts are present.

Results of the on-site salt spray corrosion tests were due to be completed in June 2017 so it would be very interesting to see the results.

An article in The Guardian focuses on what appears to be increasing opposition to the entire idea of a bridge and captures some of the key issues. If you visit the planning consent website, it's clear there are numerous objectors.

One that's particularly worth reading is from Cornish bard, Bert Biscoe, arguing that whatever the merits of the particular bridge design, they cannot outweigh the damage that will be caused to a site of major archaeological importance. The argument is not about the physical impact of the bridge, but about the very desire of the site's custodian, English Heritage, to increase visitor numbers in such a sensitive site. This is an argument about the merits of preservation over the merits of public access - it is intrinsically anti-populist, but perhaps necessary.

I have been to Tintagel and my initial feeling about the bridge was that the improved accessibility would be very welcome. The promontory is currently accessed via a low-level bridge and a series of awkward steps, which are very difficult for some visitors to traverse. The planning submission notes that some 15% of visitors who buy a ticket for the Castle never actually make it up the existing steps onto the promontory.

However, the bridge is no panacea for this, as there will still be areas which are only accessible via steps or very narrow paths. It's therefore legitimate to consider whether the adverse impacts of such a major intervention are justified by the benefits.

I will be very surprised if the bridge fails this initial planning consent hurdle. However, Scheduled Monument consent will also be required, and I expect opponents of the scheme will petition central government to call in the entire planning application for further review, such is the sensitivity of the site

I think the designers involved have done an excellent job in addressing the site constraints, within the limits of their brief, and this will be a very interesting project to follow, especially if it proceeds all the way to be built.

27 June 2017

I've posted links below to sites where you can find more information about the entries. I've only visited two, Merchant Square Footbridge and Greenwich Reach Swing Bridge. Most of the others are new to me. In general, there are some interesting bridges here, please share opinions via the comments if you want to!

21 June 2017

I've been thinking for some time about writing a few articles about unbuilt bridges, but it turns out there's so much good material elsewhere on the internet that I'm better off directing you appropriately.

The best online resource for this is the "Ian Visits" blog, which has uncovered a cornucopia of unbuilt London architecture, including bridges.

The Victorians were also responsible for an 1852 proposal to run a railway viaduct not across the Thames, but along it. This structure was proposed to run between Westminster and London Bridge, and can be seen as a forerunner of what is now the District Line, built in a tunnel below the north bank of the Thames.

At the end of the 18th century, a number of proposals were invited for the replacement of Old London Bridge. The scheme eventually selected would not be completed until 1831, but some of the alternative designs are interesting. Thomas Telford proposed a 600-foot span cast iron arch (pictured), which would have been a tremendous technological achievement if built. Creating suitable foundations to withstand the arch thrust in London Clay would have been a significant challenge! "Ian Visits" also discusses George Dance's proposal for two parallel bridges.

Thomas Dunn was the engineer behind a proposal for a remarkably modern looking iron footbridge at Ludgate Hill in 1863. Before traffic signals brought order to the city streets, Dunn's design was for a walkway-in-the-air, allowing pedestrians to cross a busy road junction without the inconvenience of having to cross overcrowded and dangerous highways.

Over the years, there have been various proposals for a new bridge across the Thames near St Paul's Cathedral (see my previous posts for some examples). "Ian Visits" documents a 1909 proposal for a road bridge, and the amusing 1997 design from FAT for a kitsch garden bridge in tribute to the late Princess of Diana (pictured). This one was ahead of its time, I think, the market for ironic monuments to celebrity has surely expanded further over the last two decades.

There have been plenty of proposals for inhabited bridges across the Thames over the years (again, some of which I've covered inthepast). It's a perennial theme from incurable nostalgicists blithely unconcerned with the visual impact of what they design. One such proposal was the Crystal Span Bridge, a 1963 proposal to replace the Vauxhall Bridge with a seven storey monstrosity complete with gallery, roof garden and ice rink.

12 June 2017

This is the third and last in this latest series of posts on the bridges of London.

Vauxhall Bridge sits on the site of a pre-Roman structure within the River Thames, only discovered in 1993 when shifts in the riverbed exposed its wooden piles. From that structure's demise until 1816, the only crossing of the river at this point was via ferry.

In 1806, engineer Ralph Dodd campaigned to build a new bridge at the Vauxhall site, leading three years later to the passing of an Act of Parliament and the creation of the Vauxhall Bridge Company. Dodd's 13-arch bridge design was, however, rapidly dropped in favour of a 7-span stone arch design by John Rennie. This then proved too expensive, and approval was obtained via a further Act of Parliament to construct an iron bridge instead. Rennie proposed an 11-span iron bridge, but a 9-span cast iron proposal from Samuel Bentham was preferred.

In its turn, Bentham's design also fell out of favour, and the bridge eventually completed in 1816 comprised nine cast iron arches supported on stone piers, designed by James Walker. This bridge proved highly successful, and was eventually taken into public ownership in 1879. However, by this time, the bridge foundations were in increasingly poor condition, and in 1895, permission was granted to replace the structure.

The new bridge also went through several incarnations before being built. Alexander Binnie's initial steel design was not popular, and he then proposed a 5-span granite-clad concrete structure. Work began on this, but problems during construction of the foundations led to a change in plan: it was decided that the concrete bridge was too heavy. The granite-clad concrete foundations and piers were completed, but Binnie and Maurice Fitzmaurice designed a steel arch superstructure instead, which was finished in 1906. This is the bridge that can be seen today.

The new bridge attracted further controversy even before it was complete. Complaints about the engineer-led design led to a decision to install statues on the bridge piers, four on each side of the bridge. The downstream statues by Alfred Drury represent the themes of Science, Fine Arts, Local Government and Education. On the upstream side, statues by Frederick Pomeroy illustrate Agriculture, Architecture, Engineering and Pottery.

Many users of the bridge will not even realise the statues are there: they face outwards to the river. They are visible from the river bank, but for a close-up view you either need to get into a boat or lean over the side of the bridge.

The bridge originally had wrought iron railings on each side, but these were replaced in 1973 with the squat parapets seen today. These are at least shorter than the original railings, which were some 8 feet tall! This was before the bridge achieved Grade II* Listed status, which only came in 2008, as part of a group of London bridges.

Apart from the parapets, Vauxhall Bridge is an attractive structure when viewed from the river. I think a large reason for this lies in the paintwork, which has been arranged to highlight a hierarchy of elements, with the riveted steel arch ribs in yellow, spandrel stanchions in white, and red, white, blue and yellow all used in different parts of the stringcourse and parapets.

There are thirteen arch ribs in each span, and the span dimensions vary from 45.6m in the centre span, via 44.0m in the intermediate spans to 39.8m in the end spans.

What struck me most about the bridge was its sheer width. It accommodates two footways, a two-lane cycleway, a bus lane, and a four-lane highway.

This is a major traffic artery, and there are further major highways running along the river at both ends of the bridge.

The result is a bridge that makes it easy cross the river, but which also plays its part in creating division, as it's almost easier to cross the river from north to south than it is to cross the bridge from east to west. Seen from the river, it's like a giant staple connecting the two river banks. Seen from above, it's like a giant pair of scissors, helping to subdivide London into smaller and more navigable city blocks.

06 June 2017

Here's another bridge built for London's 2012 Olympics, just a little further north-east along the same road. Footbridge L01 crosses above Ruckholt Road, while this bridge sits alongside it.

Ruckholt Road Footbridge is a sibling of Olympic Park Bridge 1, and it's interesting to note the similarities and differences. Both were designed by Knight Architects, although working with different engineers in each case.

Both siblings solve a common problem: to provide additional pedestrian/cyclist capacity alongside an existing highway bridge which is severely constrained in width, while spanning over a railway line. Not just any railway line, but part of the Channel Tunnel Rail Link, necessitating super-high barriers so that drunken parapet hurdlers cannot access the railway (although pole vaulters still could).

In both cases, the solution is a half-through plate girder bridge which takes as a starting point the conventional Network Rail footbridge design. In the NR design, the bridge is essentially a rib-stiffened trough, with the side panels serving as both girder webs and parapet screens, while the bottom of the trough acts as both a floor plate and as the bottom flange for the girders. Vertical stiffeners act as u-frames encompassing all three sides of the trough, to prevent the top flange from buckling under bending compression. A happy side-effect of this design is that there is no projecting bottom flange, discouraging vandals and trespassers from accessing the external elevations of the bridge.

Both Bridge 1 and Ruckholt Road designs are in weathering steel (along with several other Olympic Park bridges), reducing the need for future maintenance over what are clearly major railway lines. However, they are painted on the inside faces, where leaving the weathering steel exposed would leave it prone to damage from graffiti.

To achieve the height of screening specified by the high-speed railway authority, both sibling bridges have elevated grille screens above the level of the main girders. This creates an unpleasant tunnel effect for users, so the screens are inclined outwards along with the girders, to open up the view and make using the bridge slightly less uncomfortable.

Olympic Park Bridge 1 is a single-span bridge, and a very good design overall. However, I think that Ruckholt Road Footbridge is less successful.

The structure has two spans, a longer span over the railway with a shorter back-span over a minor road. The bridge girders have been shaped to be deeper over the main span, which I find peculiar, as a structural engineer would expect the girders to be deepest over the central support pier (where bending forces are greatest). The girder shape seems awkward when seen from the outside, although less so from the footway.

The external lines of the steelwork are clean, with the stiffening ribs looking sharp and giving the bridge an interesting visual character.

The upper parapet screens are more "tacked-on" than on Bridge 1, where their geometry was varied in an interesting manner. At Ruckholt Road, they are mostly above head height, and it's hard not to feel a little confined, even with the way the arrangement opens up towards the sky.

There are little oddities, such as the fact that the inner edge of the top flange is left unpainted, resulting in the inevitable rust staining, although this is a pretty minor impairment compared to the graffiti that adorns the girder webs.

However, the oddest thing about this bridge, for me, is to wonder why the tall screens are really needed. Here's what the existing road bridge looks like, which you can see does have footways on both sides. No effort has been made to raise the height of the parapets on this bridge, although it passes over the same railway. It's difficult therefore, to feel that the railway authority's requirements for tall parapets were genuinely necessary.

04 June 2017

I've covered most of the interesting bridges built for London’s 2012 Olympics on previous occasions, but there are two structures that I’ve been waiting for a chance to visit for some time. Now, that chance has come, and the first of the pair is the excitingly named "Footbridge L01", designed by Atkins and Allies and Morrison.

This bridge carries a foot and cycle path above the busy Ruckholt Road, a major highway in the east of London. To the south, the path leads to the Olympic Velodrome, and to the north, the playing fields of Hackney Marshes.

I don't know how well the path was used during the Olympics, but it was not a popular route when I visited, mid-morning on a sunny weekday. For a while, I wondered if I would be the only person to cross it!

The bridge spans 42m clear across the highway, and is in the form of two tied arches. The arch ribs are single steel plates, some 90mm thick, while the deck consists of ribbed steel plate (topped with concrete) spanning transversely between small box section tie girders. The tie girders are suspended from the arches by 25mm thick steel slats.

When I go to visit bridges, I often have a number of questions in mind. These are sometimes the 5 Ws of basic journalism: Who? Where? When? What? Why? When applying these to bridges they can be informative e.g. What is it actually for? The additional sixth question of "“How?" is often more useful when considering engineering and design: How does it address its purpose? How does it stand up? How does it articulate?

For me, this is a "Why?" bridge, although this question is deeply entwined with the "What?" and "How?" As originally conceived by the designers, the bridge was a tied half-through arch, with the arch ribs passing below the deck level at the bridge's ends. Why it was conceived this way, I certainly don't know, it would seem to have made more sense for the arch and tie-girder to meet at the span support points, as is normal in half-through tied arches.

It was clearly not a straightforward bridge to design, as set out in various interesting technical papers by Atkins. The close spacing of the hanger slats (at 135mm centres) means that their aggregate in-plane bending stiffness is significant, enough to act as a Vierendeel girder restraining the arch rib against in-plane buckling. This inevitably leads to complexity in the structural analysis.

Below deck level, the arch rib was originally proposed to fly clear i.e. unconnected to the deck. However, the designer’s analysis showed this would not be feasible, as the arch rib would be too slender and would buckle in compression. The obvious solution is to use the vertical slats in these end zones, but as compression struts rather than as hanger plates. Again, however, the vertical slats would buckle, as they are too slender to carry any useful compression loads.

This answers one of the bridge's several "Why"s, the question of why the end-zone elements below the bridge deck are in solid plate – this internally stiffened plate can carry vertical compression, and it's there to prevent the below-deck elements of the arch rib from buckling.

A second "Why" asks why there is a similar solid plate above deck level, but only towards the ends of the arch. There is a similar explanation – under uneven pedestrian loading, there are situations in which the slats in this zone can also experience compression load. Again, a solid plate was inserted to prevent the slats in these areas from buckling.

Before visiting the bridge, I anticipated the effect of both these engineering compromises would be awkward. This is, I think, true only to an engineering eye expecting a single clear structural rationale. I think in practice that the arch can be seen as springing not from its end supports but from the nodes where arch and tie-girder connect i.e. the main piece is held aloft in mid-air by giant orange corbels.

Another way to see these compromises positively is to see them as framing the central part of the bridge, while allowing the bulk of the structure to be visually transparent. In short, it’s an oddity, but not a complete failure.

Transparency on this bridge is only ever relative. Seen from the highway below, the hanger arrangement is reasonably transparent, with only the horizontal bar of a handrail indicating that this is as much a balustrade as a structure or a screen. While crossing the span, the bridge user exists within a local "bubble" of transparency, able to see clearly to left and right, but with their view obscured when looking at an angle. This "bubble" stays with the observer as they pass across the bridge. This louvre effect is something that can be exploited usefully where there is a need to balance issues of privacy screening and transparency, neither of which apply on Footbridge L01.

Setting aside the overall structural form for a moment, the bridge is well detailed. In particular, the approach parapets use the same slat type and spacing, albeit painted in black rather than in blazing orange. This is highly effective, and the simple continuity of the handrail element plays a positive role as well.

The substructure adopts the robust finish used elsewhere in the Olympic park: prison-style gabion baskets, here hiding a concrete support structure which is integral with the steel superstructure. Mountainous earthworks behind the grey retaining walls were hopefully used to avoid off-site disposal of spoil from excavations, as otherwise their scale is quite unnecessary.

I've left the most significant "Why?" for last, which is to ask: Why this, rather than something else?

Here I find it harder to quell a persistent nagging feeling that there was simply no especially good reason to adopt this peculiar hybrid of tied-arch and Vierendeel slats. An arch bridge is certainly an appropriate solution for the span, where a gateway structure will initially have acted as an entrance signpost to the wider Olympic site. A slender arch rib is an admirable aim, but could as readily have been achieved with, for example, a network arch design.

I suspect here that there is a degree of both affectation and of muddle: a specific notion taken to its logical conclusion, however difficult the resulting structural engineering, and a notion that is something of a muddle between architectural and engineering aspiration. There's a phrase in one of the papers on the bridge which I think may be revealing and which certainly suggests a certain kind of design relationship: "The key challenge of the design was resolving the structural integrity of the bridge without compromising the architectural vision".

Nonetheless, I enjoyed visiting Footbridge L01. In particular, the bold juxtaposition of orange and black seems quite apposite for what will for most tastes be a marmalade and marmite experience. I like the use of colour, I like the bridge detailing and I admire the willingness to depart from what might otherwise have been quite a humdrum, conventional solution.